Molecular events of muscle fiber shortening occur in fiber sarcomeres (see Figure 3). The contraction of a striated muscle fiber occurs when the sarcomeres, which are arranged linearly in the myofibrils, shorten when the myosin heads pull on the actin filaments. (3) When the signal from the nervous system is no longer present, the chemical process reverses, the muscle fibers rearrange and the muscle relaxes. In eccentric contraction, the tension generated during isometry is not enough to overcome the external load on the muscle, and the muscle fibers lengthen as they contract.  Instead of working to pull a joint towards muscle contraction, the muscle acts to slow down the joint at the end of a movement or otherwise control the repositioning of a load. This can be unintentional (e.B. when you try to move a weight too heavy to lift the muscle) or voluntarily (for example. B when the muscle “smoothes” a movement or resists gravity, by. B example during the descent). In the short term, strength training, which involves both eccentric and concentric contractions, seems to increase muscle strength more than training with concentric contractions alone.
 However, exercise-induced muscle damage is greater even with prolonged contractions.  In order for thin filaments to continue to slide beyond thick filaments during muscle contraction, myosin heads must pull, detach, tense again, attach to other binding sites, pull, loosen, spill, drop actin at binding sites, etc. This repeated movement is called the transverse bridge cycle. This movement of myosin heads is similar to that of rowing when a person rows a boat: pulling paddles from the oars (myosin heads) are lifted out of the water (detached), repositioned (rested), and then immersed again to shoot (Figure 4). Each cycle requires energy, and the action of myosin heads in sarcomeres that repeatedly pull on thin filaments also requires energy provided by ATP. When the myosin head is stretched, the myosin is in a high-energy configuration. This energy is consumed when the myosin head moves through the power stroke, and at the end of the force stroke, the myosin head is in a low-energy position. After the power stroke, ADP is released.
However, the formed transverse bridge is still present, and actin and myosin are connected to each other. As long as ATP is available, it easily binds to myosin, the transverse bridge cycle can recur, and muscle contraction can continue. In 1780, Luigi Galvani discovered that the leg muscles of dead frogs contracted when hit by an electric spark.  This was one of the first forays into the study of bioelectricity, a field that still studies electrical patterns and signals in tissues such as nerves and muscles. When oxygen is available, pyruvic acid is used in aerobic respiration. However, when oxygen is not available, pyruvic acid is converted to lactic acid, which can contribute to muscle fatigue. This conversion allows the recycling of the NADH enzyme NAD+, which is necessary for the continuation of glycolysis. This happens during intense exercise, when large amounts of energy are needed, but oxygen cannot be sufficiently supplied to the muscle. Glycolysis itself cannot be maintained for very long (about 1 minute of muscle activity), but it is useful for allowing short bursts of high-intensity performance. This is because glycolysis does not use glucose very efficiently, resulting in a net gain of two ATP per glucose molecule and per lactic acid end product, which can contribute to muscle fatigue as it accumulates. In concentric contraction, muscle tension is sufficient to overcome the load, and the muscle shortens as it contracts.
 This happens when the force generated by the muscle exceeds the load that prevents it from contracting. The sequence of events that leads to the contraction of a single muscle fiber begins with a signal – the neurotransmitter ACh – from the motor neuron, which innervates that fiber. The local membrane of the fiber depolarizes when positively charged sodium ions (Na+) enter, triggering an action potential that propagates to the rest of the membrane, including the T tubules. This triggers the release of calcium ions (Ca++) from storage in the sarcoplasmic reticulum (SR). The Ca++ then triggers a contraction maintained by the ATP (Figure 1). As long as Ca++ ions remain in the sarcoplasm to bind to troponin, which keeps actin binding sites “unshielded,” and as long as ATP is available to drive the cross-bridge cycle and myosin pulling of actin strands, the muscle fiber will continue to shorten to an anatomical limit. Duchenne muscular dystrophy (DMD) is a progressive weakening of skeletal muscle. It is one of many diseases collectively called “muscular dystrophy”. DMD is caused by a deficiency of protein dystrophin, which helps the thin filaments of myofibrils bind to the sarcolemma. Without adequate dystrophin, muscle contractions cause the sarcolemma to rupture, leading to an influx of Ca++, leading to cell damage and breakdown of muscle fibers. Over time, as muscle damage accumulates, muscle mass is lost and greater functional impairments develop. Their muscles contain fiber called myosin.
Depending on how you need to use your muscles, myosin fibers tighten and shorten or relax and expand. Myosin is also responsible for muscle contractions such as your heart rate, which occurs at regular intervals. Aerobic respiration is the breakdown of glucose or other nutrients in the presence of oxygen (O2) to produce carbon dioxide, water and ATP. About 95% of the ATP needed for rest or moderately active muscles is provided by aerobic respiration that takes place in the mitochondria. Inputs for aerobic respiration include glucose circulating in the bloodstream, pyruvic acid and fatty acids. Aerobic respiration is much more efficient than anaerobic glycolysis, producing about 36 ATP per glucose molecule compared to four from glycolysis. However, aerobic respiration cannot be maintained without a regular supply of O2 to skeletal muscles and is much slower (Figure 7). .